The verb can has two forms: can for the Present Tense and could for the Past Tense; the expression to be able to has the same meaning and can be used to supply the missing forms of the verb can.

The verb can has two forms: can for the Present Tense and could for the Past Tense; the expression to be able to has the same meaning and can be used to supply the missing forms of the verb can. There is only one future form, for can is not used in the future except to express permission. All other tenses are formed with be able according to the rules for ordinary verbs.

Function:

1. Physical or mental ability (used only with the Indefinite Infinitive)

When we say that someone or something has or doesn't have the ability to do something, we can use can('t) (or cannot) (for the present) or could(n't) (for the past):

He cananalyse people's handwriting.

Anita couldspeak three languages before she was six.

We can use be able to instead of can/could to talk about an ability that someone has or had:

Helen can / is able to read well, even though she's only three.

In general, we use be able to when we talk about a specific achievement (particularly if it is difficult, requiring some effort) rather than a general ability.

	General ability	Specific achievement
Present	We prefer can (but also we use be able to): He can speak Spanish.	We prefer be able to (but can also use can): He has now recovered from his injury and is able to drive again.
Past	We prefer could (but can also use be able to): After only six months, Suzanne could play the violin quite well.	We use be able to (not could): Jenny was able to leave the hospitalonly six hours after the baby was born.

However, we commonly use can or could, even when we are talking about specific occasions, with verbs of the senses, feel, hear, see, smell, taste, and with verbs of 'thinking', e.g. believe, decide, remember, understand:

She could feel the spray on her face as the boat raced through the water.

I can't decide where to go for my holidays.

!!! We don't use be able to

• when we talk about something that is happening as we are speaking:

Look, I can swim.

• before be + past participle:

This furniture can be assembled by anyone, with just a screwdriver.

We use be able to, not can or could, if the idea we want to express needs a to-infinitive, an –ing form, or a perfect tense, or if it follows another modal verb:

We were very lucky to be able to live in the country during our childhood.

It was awful, not beingable to see you for so long.

Since he left, none of the other team members have been able to match his enthusiasm.

We may be able to move some of the staff to a different departments.

We use will be able to, not can, to say that something will be possible in the future:

If the snow carries on like this, very few people will be able to get to the concert.

When the new road is built, I'll be able to drive to work in under half an hour.

However, when we make a decision now about something in the future, we use can:

You can go home when you've finished writing your composition.

Perhaps we can meet next week.

2. Possibility (used with the Indefinite Infinitive only)

a) due to circumstances (meaning “It is possible”):

I could not go to the theatre yesterday because I was busy.

You can ski on the hills. (There is enough snow)

Can you get to the top of the mountain in one day? (Is it possible?)

Can I have my photo taken? (Can comes close to may, expressing permission, but in this case we are interested not in the will of the person addressed but in the fact they are ready or not)

b) due to existing law:

You can’t play football in the street. (You have no right)

Children can get a passport at the age of 16.

3. Incredulity, doubt, astonishment. (used with all the forms of the Infinitive in interrogative and negative sentences, though astonishment is expressed only in interrogative sentences):

“That’s not true!” exclaimed Linton, rising in agitation. “It cannot be!”

Can she be waiting for us?

Can/could she have said that?

The car was going too fast. They couldn’t have seen us.

Could with the Perfect Infinitive has almost the same meaning as can with the Perfect Infinitive, only the negation is not strict as with can.

4. Permission in the past (used mostly with could)

Expression of general permission in the past:

On Sundays we could stay up late. (We were allowed)

We couldn’t bring our dog into the restaurant.

Can I? and could I? as informal request for permission:

Can I stay up till the end of the programme?

Can Tom use the car whenever he likes?

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